An optical modulator is an important device used to convert an electric signal into an optical signal on an optical communication network, etc. Several optical modulation methods are employed, such as a method for directly blinking emission of a light source, a method for changing the transmittance of a medium that passes light, and a method for changing interference output by controlling the optical phase of an optical interferometer. Among these methods, the method that uses an optical interferometer has a feature of that the signal spectral band width of a modulated optical signal output is narrow. As a major modulation method, the method that uses an optical interferometer is employed, especially for long distance transmission having a problem that residual dispersion causes broadening of waveforms, and for wavelength division multiplexing transmission for transmitting many wavelength channels at high density. An optical interferometric modulator in practical use consist of optical waveguides (LN waveguide) that was formed by titanium diffusion on a lithium niobate (LiNbO3: LN) substrate with large electro-optic effects. The optical phase control is performed by applying the voltage of an electric signal to electrodes located in the vicinity of the waveguide.
For a transmission rate of up to about 10 Gbps for a wavelength channel, a modulated signal by on-off intensity keying with a binary-value basis is used. For transmission at a higher rate beyond 10 Gbps, a multi-level modulated signal provided by using phase information as well as the amplitude of the optical signal, and/or a polarization-multiplexed signal provided by multiplexing a signal using polarization, is employed in order to efficiently transmit multiple bits with one symbol. For transmission performed at 40 Gbps, Differential Quadrature Phase-Shift Keying (DQPSK) has already been practically used, whereby data consisting of two bits can be transmitted using one symbol. For transmission at the rate of 100 Gbps for which practical use is anticipated, polarization-multiplexed QPSK that employs polarization multiplexing is also being studied as one of the major candidates. A difference between QPSK modulation and DQPSK modulation is that in QPSK modulation, a sign is allocated for the phase value of each symbol, while in DQPSK modulation, a sign is allocated to a phase change value with respect to a preceding symbol. However, from the viewpoint that signals are modulated and assigned to four optical phases, these two techniques are alike, and the configuration of a modulator and the physical modulation method employed for both these techniques do not differ.
FIG. 1 illustrates the configuration of a conventional optical modulator. Here, to perform the high-speed signal transmission described above, RZ (Return To Zero) pulse carving, for isolating/standardizing the intensity-modulated waveforms of the individual symbols, is frequently employed, based on the viewpoint that the suppression of nonlinearity be performed during signal transmission and that the suppression of chirp be performed during inter-symbol transition. As shown in FIG. 1, in a conventional optical modulator 100, a DQPSK modulator 110 and an RZ pulse carver 120 are connected by a polarization-maintaining optical fiber 132.
The DQPSK modulator 110 is a modulator having a nested structure (a nested MZI modulator), wherein an I-channel MZI modulator 113 and a Q-channel MZI modulator 114 are respectively inserted into two main arm waveguides held between a 1×2 coupler 111 and a 2×1 coupler 112. A π/2 phase shifter (a variable phase shifter) 115 is inserted into at least one of the two arm waveguides. The I-channel MZI modulator 113 and the Q-channel MZI modulator 114 are common Mach-Zehnder interferometric modulators, respectively, wherein two individual arm waveguides held by a 1×2 coupler and a 2×1 coupler include a phase shifter for modulation. The phase shifter for modulation provided for the I-channel MZI modulator 113 is driven with a data signal (a Data I signal), and the phase shifter for modulation provided for the Q-channel MZI modulator 114 is driven with another data signal (a Data Q signal).
The RZ pulse carver 120 is a common single Mach-Zehnder interferometric modulator (MZI modulator), wherein phase shifters 123 and 124 for modulation are respectively provided for two arm waveguides held by a 1×2 coupler 121 and a 2×1 coupler 122. The phase shifters 123 and 124 for modulation are driven with a clock signal (a CLK signal).
A continuous wave (CW) light is input at an input optical fiber 131 connected to the DQPSK modulator 110, and an RZ-pulse-format DQPSK signal (an RZ-DQPSK signal) is output by an output optical fiber 133 connected to the RZ pulse carver 120.
While referring to FIGS. 2A to 2D, the operating principle of the Mach-Zehnder interferometric modulator will be described. Here, it is assumed that the modulator is an LN modulator formed by using a Z-cut substrate; however, when an X-cut substrate is employed for a modulator, basically the same operation is performed. An MZI modulator shown in FIG. 2A includes phase shifters 153 and 154 respectively provided for two arm waveguides that are held by a 1×2 coupler 151 and a 2×1 coupler 152. When a Z-cut substrate is employed, a so-called push-pull operation is performed, wherein a drive electrical signal Vdrv is input, as +Vdrv/2 to the phase shifter 153 for modulation and as −Vdrv/2 to the phase shifter 154 for modulation. It should be noted that when an X-cut substrate is employed, a drive electrode is arranged between the two arm waveguides. When a drive electrical signal is applied to the electrode, an electric field is applied, in opposite directions from each other, to the upper and lower phase shifters for modulation, and that therefore, a push-pull operation is automatically performed.
The input CW light is split into two beam lights by the 1×2 coupler 151, the lights are modulated in phase by the phase shifters 153 and 154 respectively of the two arm waveguides, and the lights are coupled by the 2×1 coupler 152. At this time, the phase of the output optical signal in the electric field changes, as shown in FIG. 2B. Then, since the light transmitted via the phase shifter 153 is affected by positive phase modulation, the trajectory of the field vector is counterclockwise (x→∘→●) (2-1). Whereas, the light transmitted via the phase shifter 154 is affected by negative phase modulation, and the trajectory of the field vector is clockwise (x→∘→●) (2-2). The field vector of the output optical signal is obtained as resultant vector of these vectors, and thus, the trajectory of the output optical signal is a linear trajectory along the real axis (2-3).
At this time, when the phase shifters are driven with a Data signal to change a phase difference for the arm waveguides by 2π, as shown in FIG. 2C, the phase of the output light is changed to phases 0 and π. The intensity of the signal for the single MZI modulator is unchanged, and the modulator serves as a phase modulator that outputs two phase values.
FIG. 3 shows the intensity waveform of an optical signal and the phase of the optical signal in the electrical field according to the conventional optical modulator. When CW light is input to the optical modulator 100 in FIG. 1 (1-1), the CW light is split by the 1×2 coupler 111 (1-2 and 1-3), and the split lights are transmitted to the I-channel MZI modulator 113 and the Q-channel MZI modulator 114. As shown in FIG. 2C, the modulated optical signals (1-4 and 1-5), which have two phase values are output by the I-channel MZI modulator 113 and the Q-channel MZI modulator 114. In this drawing, to simplify the explanation, the same modulation pattern “1 0 0 . . . ” is employed both for the I channel and the Q channel; however, for the actual modulation operation, the same pattern is not always employed. The modulated signals are coupled, with a phase difference of 90° (1-6), while a quarter wavelength is employed as the optical path difference between the I channel and the Q channel. As a result, as shown in (1-7), a QPSK optical signal modulated using four phase values is obtained as an optical signal to be output. It should be noted that for QPSK modulation performed by the nested MZI modulator, a fundamental loss of 3 dB occurs due to the coupling process performed with a phase difference of 90°.
When the phase shifters are driven with a CLK signal to change the phase difference of the arm waveguides by π, as shown in FIG. 2D, the phase of the output light is unchanged. The single MZI modulator serves as a pulse carver for generating a solitary pulse, for which each of the intensity-modulated waveforms of the pulses in an optical signal is uniform. Therefore, when this pulse carver is employed as the RZ pulse carver 120 of the optical modulator 100 in FIG. 1, as shown in (1-8) in FIG. 3, the individual symbols can be formed as solitary pulses having the same intensity-modulated waveform, while the optical phase information for the DQPSK modulated optical signal is maintained. It should be noted that the RZ pulse carving performs waveform shaping by attenuating an optical waveform, so that the reduced portion of the waveform is regarded as a fundamental loss. In a case wherein waveform shaping is performed to obtain a RZ pulse at a duty ratio of 50%, a fundamental loss of 3 dB occurs.
For an LN waveguide, a phenomenon called “DC bias drift” occurs that, when a voltage is being applied for a long time, a charge-up phenomenon and the like occurs and causes the refractive index of the waveguide and an interference condition to be shifted. Further, a phenomenon called “temperature drift” occurs by which the refractive index is also shifted, due to an environmental temperature. For the MZI modulators, such as the child MZI modulators (the I-channel MZI modulator 113 and the Q-channel MZI modulator 114 in FIG. 1) in the nested structure and the RZ pulse carver, shifting of the interference condition appears as a shift of an operating point. For the parent MZI of the nested MZI modulator, a relative phase shift in orthogonality for the I-channel/Q-channel optical signals appears, i.e., a shift from a phase difference of 90°. Since these shifts are undesirable, because the quality of the optical signal is deteriorated, it is required that appropriate monitoring means be employed to detect, and compensate for/adjust the amount of shift.
To perform compensation for the shift of the operating point of the MZI modulator, an electrical circuit that combines a high frequency signal component, called a Bias-T, with a direct-current bias component is inserted at the front stage for high frequency input, and a bias voltage is applied to a modulated signal. As another method, a dedicated bias electrode, for compensating for/adjusting the operating point, is provided separately from the high frequency electrode, and a bias voltage is applied to the bias electrode to perform compensation.
To perform a 90° phase adjustment using the parent MZI, a variable phase shifter 115 arranged in the parent MZI in FIG. 1 is employed to adjust the relative phase for the I-channel/Q-channel optical signals.
To monitor a shift of the operating point and a shift in orthogonality, generally, a monitor output branched at an optical tap and the like is provided for the output port of each modulator to monitor the optical output. Specifically, the monitoring device for the DQPSK is arranged at the rear of the 2×1 coupler 112, and the monitoring device for the RZ pulse carver is arranged at the rear of the 2×1 coupler 122.
However, for RZ pulse carving as described above, since an optical signal passes the MZI modulators connected at multiple stages, an insertion loss is increased, and a fundamental loss of about 3 dB occurs while RZ pulse carving is being performed. Therefore, a problem arises that the intensity of the modulated optical signal is significantly attenuated.